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24
CHAPTER 3
Interpretation, Appraisal, and Application
The primary objective of NCHRP Project 4-19(2) was to because the CA-3 (Uncrushed Gravel) mixture has no data at
validate performance-related aggregate tests recommended that point. The matrix is shown in Table 20. The correlations
by Kandhal and Parker (1). A discussion of the relationships between the rutting parameters and the coarse aggregate
of these aggregate test methods to mixture performance and UVA and UVB are significant. Each of the test methods was
aggregate properties follows. assigned a descriptive ranking as shown in Table 20. The rank-
ing was determined by assigning a value from 0 to 9 depending
on the correlation coefficient between the given test method and
Coarse-Graded Mixtures
the rutting parameters. For example, if the correlation coeffi-
Kandhal and Parker recommended the coarse aggregate cient for a given test was higher than 0.80, but less than 0.90, the
UVA (AASHTO TP 56) and the FOE21 as the first and second descriptive ranking was assigned as 8. The ranking for a given
best coarse aggregate tests related to rutting, respectively. test method was then averaged for the rutting parameters used.
They also found that FOE51 was an important variable, but it Thus the higher the ranking number, the better the aggregate
was not recommended because of the narrow range in results test relates to HMA rutting performance in the APT. Table 20
and did not provide a clear measure of a coarse aggregate's shows that the coarse aggregate UVA and UVB have the highest
performance. descriptive rankings, 9.0, while the FOE21 has the next closest
The regression equations developed by Kandhal and at 8.0. The remaining aggregate tests have much lower rankings.
Parker (1) suggested that as UVA increases, both rutting and Plots of the three permanent deformation parameters
rate of rutting decrease. An increase in FOE21 causes a and the coarse aggregate UVA values are shown in Figures
reduction of mixture stiffness and an increase in the rate of 16 through 18. These figures suggest that higher coarse
rutting. In summary, Kandhal and Parker (1) suggested that aggregate UVA values result in greater resistance to perma-
high coarse aggregate UVA and low FOE21 values are desir- nent deformation. The plots also indicate that continued
able for improved pavement performance. The current increase in rut resistance becomes negligible for UVA values
research also found a strong correlation between FOE21 and higher than approximately 50 percent.
coarse aggregate UVA (r = 0.786, p-value = 0.064). The cor- Although the flat or elongated and flat and elongated values
relation suggests that the FOE21 value significantly influ- at the 3:1 ratio (FOE31and F&E31) were expected to correlate
ences the orientation and structure of coarse aggregate as positively with FOE21, the data shown in Table 20 indicated
reflected by the UVA values. that neither of the 3:1 ratios correlates well to the rutting
Because the two variables, coarse aggregate UVA and parameters. The relationships between rutting and FOE31 or
FOE21, are highly correlated, it is not appropriate to use both F&E31 suggest that factors other than particle shape contribute
as independent variables in a statistical model. Moreover, the to explaining rut resistance. Coarse aggregate UVA appears to
conclusion drawn by Kandhal and Parker (1) about the effect successfully capture the effect of particle shape, surface charac-
of FOE21 on pavement permanent deformation can become teristics, or mineralogy on rutting performance of the HMA
unrealistic. In fact, a high percentage of FOE21 aggregate par- mixtures tested in this research.
ticles are desirable for HMA pavements. To further investigate interaction of aggregate tests, multiple
For the current study, a correlation matrix was developed for regression analyses were performed between rutting parame-
eight of the coarse aggregate tests and three rutting parameters. ters and coarse aggregate test data. The response variables
The total rut depth at 20,000 wheel passes was not included included total rut depth at 5,000 wheel passes and total rut rate